An electrophotographic image forming apparatus develops an electrostatic latent image formed on an image carrier using developer, and transfers the developed image onto a print medium, thereby forming a predetermined image on the print medium.
Developing apparatuses using two-component developer including toner and carrier as the developer for developing the electrostatic latent image are used.
Such a developing apparatus includes a first mixing member for mixing the developer and a second mixing member for supplying the developer to the image carrier. The developing apparatus is required so that the second mixing member stably supplies the developer to a developing roller. For this it is good that a height of the developer in a second developer area in which the second mixing member is disposed is maintained higher than a height of the developer in a first developer area in which the first mixing member is disposed.
When the height of the developer in the second developer area is low, the second mixing member cannot supply a sufficient amount of the developer to the developing roller. At this time, as illustrated in FIG. 1, an image density deviation of a vertical direction called as auger mark 110 is generated in a period of a pitch p of the second mixing member 100.
Also, when the height of the developer in the second developer area is too high, the developer supplied from the second mixing member to the developing roller is moved with rotation of the developing roller, separated from the developing roller, and falls toward the second mixing member. However, before the developer is mixed by the second mixing member, some developer is reattached to the developing roller. When this phenomenon occurs, replaceability of the developer is deteriorated. In the case in which the replaceability of the developer is deteriorated, when documents of high coverage are continuously printed, the image density is gradually lowered.
In order to prevent generation of the auger mark, a method of increasing a diameter of a shaft of the second mixing member than a diameter of a shaft of the first mixing member has been proposed. However, if the diameter of the shaft of the second mixing member is increased, the height of the developer of the second developer area is maintained high, but mixability of the developer by the second mixing member is weakened. When the second mixing member does not sufficiently mix the developer, scattering of the developer occurs.
Recently, use of an auto developer refill developing apparatus which supplies the developer with toner to which a small amount of carrier is added and discharges surplus developer is increasing. The auto developer refill developing apparatus is mainly configured so that a developer discharge port is provided at an end portion of the mixing member, and when the height of the developer in the vicinity of the developer discharge port is above a certain value, the developer overflows to be discharged.
The height of the developer is changed by the rotational speed of the mixing member. By the way, a printing speed may be lowered in accordance with printing conditions. For example, when printing a high-resolution image, or when printing a thick paper, the printing speed is lowered. At this time, the printing speed is often approximately half of the normal speed (maximum speed). Accordingly, the low-speed is generally referred to as a half speed.
When the printing speed is changed as described above, the rotational speed of the mixing member of the developing apparatus also is changed according to the printing speed. At this time, the change in the height of the developer is very large. As fast as the rotational speed of the mixing member of the developing apparatus is, the height of the developer is increased so that a lot of developer is discharged. However, when the rotational speed of the mixing member is slow, the height of the developer is lowered so that the developer is not discharged. In other words, when the rotational speed of the mixing member is fast, the amount of developer decreases, and when the rotational speed of the mixing member is slow, the amount of developer increases.
When the printing speed is repeatedly switched between the maximum speed and the half speed, a case in which the amount of developer is small and the height of the developer is low and a case in which the amount of developer is large and the height of the developer is high occur. When the height of the developer is low in the second mixing member including the second mixing member and the developing roller, the image density deviation called as auger mark is generated. On the other hand, when the height of the developer is high, the replaceability of the developer is deteriorated so that the image density is lowered. Accordingly, in the auto developer refill developing apparatus, it is preferable that even when the printing speed is changed, the amount of developer is not changed.